n comms 3435

ARTICLE

Received 3 Sep 2012 | Accepted 12 Aug 2013 | Published 10 Sep 2013

Carbon nanotubes on a spider silk scaffoldEden Steven1, Wasan R. Saleh2, Victor Lebedev3, Steve F.A. Acquah4, Vladimir Laukhin5, Runa G. Alamo6

& James S. Brooks1

Understanding the compatibility between spider silk and conducting materials is essential to

advance the use of spider silk in electronic applications. Spider silk is tough, but becomes soft

when exposed to water. Here we report a strong afnity of amine-functionalised multi-walled

carbon nanotubes for spider silk, with coating assisted by a water and mechanical shear

method. The nanotubes adhere uniformly and bond to the silk bre surface to produce tough,

custom-shaped, exible and electrically conducting bres after drying and contraction. The

conductivity of coated silk bres is reversibly sensitive to strain and humidity, leading to

proof-of-concept sensor and actuator demonstrations.

DOI: 10.1038/ncomms3435 OPEN

1 National High Magnetic Field Laboratory, Department of Physics, Florida State University, 1800 East Paul Dirac Drive, Tallahassee, Florida 32310, USA.2Department of Physics, College of Sciences, University of Baghdad, Baghdad 10071, Iraq. 3 Institut de Ciencia de Materials de Barcelona (ICMAB-CSIC)/CIBER-BBN, Campus UAB, Bellaterra 08193, Spain. 4 Department of Chemistry and Biochemistry, Florida State University, 95 Chieftan Way, Tallahassee,Florida 32306, USA. 5 Institucio Catalana de Recerca I Estudis Avancats (ICREA), Barcelona 08010, Spain. 6Department of Chemical and BiomedicalEngineering, FAMU-FSU College of Engineering, 2525 Pottsdamer Street, Tallahassee, Florida 32310-6046, USA. Correspondence and requests for materialsshould be addressed to E.S. (email: [email protected]).

NATURE COMMUNICATIONS | 4:2435 | DOI: 10.1038/ncomms3435 | www.nature.com/naturecommunications 1

& 2013 Macmillan Publishers Limited. All rights reserved.

The immense demand for electronics, and thus theelectronic waste and environmental pollution it generates,poses a growing problem that will require innovative

solutions1. Many toxic elements and non-biodegradable plasticsare commonly found in conventional electronics, and efforts todevelop new eco-friendly electronic designs are thereforedesirable. Incorporation of natural materials into these designsis advantageous to reduce the quantity of toxic components of theelectronic devices. Moreover, natural materials often possesscomplex and robust physical properties that can be harnessed forelectrical and sensor applications. Spider silk (SS) is one suchmaterial and the combination of its toughness2 and bio-compatibility3,4 makes the material strategically important forimplant, electrical, sensor and actuating applications.SS, a protein-based natural polymer, is a exible but strong

material due to its helical-elastic and b-sheet crystallinecomposition5,6. An unrestrained neat SS bre expands in bothlength and diameter7,8 when humidied up to B70 or 80%relative humidity (RH). At higher RH, the bre experiencessupercontraction710, where it shrinks in length, expands indiameter and becomes soft. This bre shrinkage is typically anirreversible process11. The bre softening, however, is a reversibleprocess12. In addition, the bre also experiences cycliccontraction11, a phenomenon different from supercontraction,where the bre extends when exposed to a high-humidityenvironment. These factors are key to the work presented here.For technological applications, where constant strength

and exibility in a variable environment are desired, super-contraction may be regarded as a problem. However, bothsupercontraction and cyclic contraction can be exploited foractuating applications. For example, it has been shown that SSbres can be used as a biomimetic muscle with an exceptionalwork density, 50 times higher than other biological musclebres, estimated to be capable of lifting a 5 kg mass with a1mm thick SS bre11. SS bres can also be used as contact13

or shadow14 masks during thin lm deposition, generatingmicro-13 or nano patterned14 features without lithographicprocessing. Moreover, starting from its intrinsic properties, SSbres can serve as a versatile scaffold upon which additionalfunctions can be built. For example, CdTe15, magnetite16 andgold16,17 nanoparticles can be used to functionalise SS foruorescent, magnetic and electronic applications, respectively.Gold-functionalised bres (Au-SS) have been shown to beelectrically robust down to cryogenic temperatures17. Eventhough Au-SS possesses sufcient exibility for use aselectrodes in microelectronics17, generally its elasticity andelectrical continuity are not adequate for electronic sensors oractuating devices.Here we show that supercontraction, and in particular, silk

bre softening, provides a simple and effective route of SSfunctionalisation with carbon nanotubes (CNTs), enabling usein electronic applications including sensors and actuatingdevices. We report a strong afnity for amine-functionalisedmultiwall CNTs (f-CNTs) to adhere to natural Nephila clavipesSS bres. Adhesion is facilitated by water and mechanical shear,and enhanced by polar interactions and bonding between the SSand f-CNT side groups. The process results in SS bresuniformly coated with f-CNTs (f-CNT-SS) providing anelectrically conducting path, and thereby a self-monitoringmechanism for physical changes and/or stimuli to the f-CNT-SS structure. The f-CNT-SS bres are B300% tougher thanneat silk bre, versatile and multi-functional, and exhibitpolar (Supplementary Movie 1), shapeable, conducting, exible,strain- and humidity-sensitive properties. Proof-of-conceptf-CNT-SS-based heart pulse sensor and current-driven actuatordevices are demonstrated.

ResultsWater-based f-CNT coating of SS bres. We discovered that bymixing a bundle of dragline SS bres (B2 cm long) with a drypowder of f-CNTs (Methods section), applying a few drops ofwater, and then pressing and shearing the mixture between twoTeon (Polytetrauoroethylene) sheets, the bres turned veryblack, and when dried, contracted to a well-dened geometrywhere the silk bres were uniformly coated with nanotubes(Fig. 1). The neat bundle contained multiple dragline silk bres intheir natural double-stranded arrangement (each strand has adiameter of B4 mm), all of which were coated simultaneously.After the coating process, the dragline silk bres were wellseparated into individually coated single-strand bres (referred toas single bres for the rest of the paper), accompanied by smallisolated f-CNT aggregates (Supplementary Fig. S1).This separation allowed reliable extraction of single silk bres

from the bundle. SEM and TEM images of the single silk breshow that the f-CNTs are attached to the SS structure (Fig. 2ad),including some penetration of the nanotubes into the SS surface(Fig. 2e). This procedure produces a basic uniform annularf-CNT coating with thickness of B80100 nm with occasionalf-CNT aggregates of B1 mm in diameter and thickness(Supplementary Fig. S2). Additional SEM and TEM images ofanother silk bre are available (Supplementary Figs S1 and S2).We have also performed a control experiment involving pre-

supercontracted bres. The neat bres are rst immersed in awater bath for 30min, followed by air drying, and then the water-based f-CNT coating is performed. The water-based procedure isalso effective on these pre-supercontracted bres, indicating thatthe initial shrinkage of silk bre is not the most important factorto achieve the effective coating, but it is the softening of the breduring supercontraction.We note that a dry powder of pure multiwall CNTs

(MWCNTs) does not provide effective initial dispersion andadhesion to the SS bre (Supplementary Fig. S3). As a result, it isnot possible to coat the SS bre with pure MWCNTs using ourwater-based method. Likewise, only SS bres exhibit an effectivef-CNT coating compared with nylon, polyester, cotton and someacrylic bres where either spotty or no coating was observed.Unlike water, other solvents such as hexane, toluene, methanol,ethanol, acetone, dichloromethane and dimethylsulphoxide donot facilitate a uniform coating.

Fourier transform infrared spectroscopy. To examine moreclosely the nature of the f-CNT/SS interface, and in particular theinteraction between the NH2 side group of the f-CNTs and SS

Dryf-CNT-SS

bundle

Press and shear

WetMix

Figure 1 | f-CNT-SS processing steps. A neat SS bundle is mixed with a dry

f-CNTpowder. The mixture is then wet, pressed, sheared between two PTFE

substrates, and air dried. Scale bar: 2mm.

ARTICLE NATURE COMMUNICATIONS | DOI: 10.1038/ncomms3435

2 NATURE COMMUNICATIONS | 4:2435 | DOI: 10.1038/ncomms3435 | www.nature.com/naturecommunications


protein structure, Fourier transform infrared (FTIR) spectro-scopy is employed. The overall FTIR spectra of both the neat SSand f-CNT-SS are similar (Fig. 3a), showing almost identicalfeatures in the amide I (1,7001,600 cm 1) and amide II(1,5001,400 cm 1) region18,19. As the absorption in amide Iand amide II regions is sensitive only to the secondarystructure of the SS bres19, this indicates that there are nomajor secondary structure transformations after the f-CNTcoating; therefore, the b-sheet and coil composition in the SSprotein structures remain the same.However, a distinct feature in the amide III (1,350

1,200 cm 1) region18,19 is observed. Contrary to the amide Iand II regions, the absorption in the amide III region issensitive to the structure of amino acid side-chains19. InFig. 3c, a peak intensity reduction is observed for theabsorption corresponding to the O-H bond of aspartic andglutamic acid (1,254 cm 1)19, indicating transformations of afraction of the acid hydroxyl groups. The content of O-Hgroups subjected to the transformations is extracted fromdeconvolution of the bands, as shown in SupplementaryFig. S4. The reduction is very subtle (Supplementary Fig. S5),as expected, due to the low abundance of aspartic andglutamic acids in SS bres (maximum abundance of B0.6and B8.8%, respectively)20. Additional evidence for chemicalinteractions is revealed by the change in the C-H symmetricstretching absorption prole observed in 2,9002,800 cm 1region (Fig. 3b), where the peak intensities at 2,851 (CH2stretch) and 2,872 cm 1 (CH3 stretch) are reduced andincreased, respectively, after the CNT coating21,22. How theacid hydroxyl group transformation affects the C-H stretchingabsorption is not yet understood. As a control, we havealso compared the FTIR spectra of neat bres with bresthat have been subjected to water, shear and pressure withoutthe f-CNTs. We conrm that no observable difference inthe FTIR spectrum is observed after the treatment(Supplementary Fig. S5).

Raman spectroscopy. Raman spectroscopy has been used toevaluate the effects of the water-based method on the propertiesof the f-CNTs being used. In Fig. 4, the Raman spectrum of asingle f-CNT-SS bre is compared with the spectrum of a drypowder of the f-CNT control sample. Owing to the thin f-CNT

Figure 2 | Single f-CNT-SS bre surface proles. (a) SEM image of f-CNT-SS. The diameter of the bre is B6.5 mm. Scale bar: 10mm. (b,c) MagniedSEM images of f-CNT-SS surface showing a uniform mat-like covering. Scale bars: 1 mm. (d) TEM cross section of a dragline silk bre with f-CNTcoating (red arrow). Silk folding due to uneven sectioning. Scale bar: 1 mm. (e) TEM image indicating nanotube penetration (red arrows) into the silkstructure. Scale bar: 250 nm.

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Figure 3 | FTIR spectra of neat and f-CNT-SS bres. (a) Combined FTIR

spectra normalised to the amide I peak at 1,625 cm 1 (red: neat bres,blue: f-CNT-SS). (b) Expanded view of the spectra between 3,000 and

2,800 cm 1. Arrows indicate the position of the peaks of interest.(c) Expanded view of the spectra near amide III region. A subtle reduction in

the peak intensity near 1,260 cm 1 is observed after f-CNT coating.

NATURE COMMUNICATIONS | DOI: 10.1038/ncomms3435 ARTICLE



coating, a weak background signal that comes from the SS bre(Supplementary Fig. S6) is observed in the spectrum. Except forthe partial contribution of the background signal, both thef-CNT-SS bre and the control sample exhibit similar D-band(B1,300 cm 1) and G-band (B1,600 cm 1) peaks, which areassociated with disordered and graphitized carbon, respectively.With the SS background subtracted, the normalized spectrum off-CNT-SS is almost identical to that of the control (inset ofFig. 4), including the D-band/G-band peak intensity ratio, indi-cating that no additional disorder is introduced during thecoating process. The uniformity of the f-CNT coating is con-rmed by the similarity of the Raman spectra for three randomareas of the same bre (Supplementary Fig. S6).

Tensile properties. The strength of the f-CNT-SS bre (B0.6GPa) is lower than that of a neat bre (B1.4GPa), but is verysimilar to the strength of the supercontracted dry bre (B0.6GPa), as shown in Fig. 5. However, the f-CNT-SS bre has alarger extensibility (dened as the maximum strain allowedbefore breaking) of B73%, compared with that of the neat(B16%) and dry supercontracted (B47%) bres. Therefore, thetoughness (dened as the area under the stressstrain curve) off-CNT-SS bre is improved to B290MJm 3, B300% morethan that of a neat bre. A summary of the tensile properties ofthe bres is presented in Supplementary Tables S1 and S2.

Electrical conductivity. The conductivity extracted from theresistance of a single f-CNT-SS is in the range of 1215 S cm 1(see Methods for calculation details). This value is consistent withthat of a buckypaper control sample (B13 S cm 1) made usingthe same f-CNT powder with benzoquinone cross-linkers fol-lowing the procedure described in the study by Ventura et al.23

The single f-CNT-SS conductivity value is also consistent with thereported value24 for MWCNT array samples of 714 S cm 1.

Custom shaped f-CNT-SS. As shown in Fig. 6a, it is possible toshape the f-CNT-SS into various forms while it remains con-ducting. The f-CNT-SS is rst wet using de-ionized water andbent into the desired shape. After drying, the f-CNT-SS shape ismaintained and the conductivity stays within 20% of its originalvalue. This process can be repeated to obtain other shapes withsimilar conducting properties. The softness of f-CNT-SS while inthe wet state may mitigate the formation of CNT cracks duringthe bending process.

Temperature-dependent resistance. The shapeable property off-CNT-SS allows us to make clamped or woven electrode con-gurations for electrical measurements. Electrodes can be madeby wetting the f-CNT-SS bres, and then winding and weavingthem onto a four-terminal copper wire resistance circuit (Fig. 6b).After drying, the bres contract, squeezing the outer two copperwires (current leads) and increasing the tension between the innertwo copper wires (voltage leads). The contracted f-CNT-SS/cop-per electrical contacts are robust in ambient or vacuum envir-onment down to cryogenic temperatures. The resistance (R)follows a three-dimensional (3D) variable range hopping(VRH)25 behaviour RR0 exp ([T0/T]1/4) down to B4.3 K(Fig. 6c), with a barrier energy T0B108K, consistent with that ofa pressed pure-MWCNT pellet reported in the literature26. Thisbehaviour is compared with that of a buckypaper control sample(Supplementary Fig. S7). At higher temperature, T430K, thebuckypaper follows a 3D VRH behaviour with T0B53K, com-parable with the case of the f-CNT-SS. However, at o30K, thebuckypaper exhibits one-dimensional (1D) VRH behaviour, forexample: RR0 exp ([T0/T]1/2).

Strain-dependent resistance. Under a strain Dl/l of up to at least50% (Fig. 7a), a single f-CNT-SS bre remains conducting, andthe f-CNT coating can expand and contract in unison with the SSbre, thereby maintaining an electrical linkage without crossing apercolation threshold. In some cases, it is possible to obtain bresthat in the dry state under ambient conditions can be stretchedup to 200% of the initial length while remaining conducting(Supplementary Movie 2; a bundle of bres was used in the videofor clear visualization). With a gauge factor (DR/R)/(Dl/l)1.20.02 (Fig. 7b), this leads immediately to proof-of-conceptapplications such as strain-sensitive resistive sensors for heart-pulse monitoring (Fig. 7c). The f-CNT coating also provideselectrical readout to monitor strain changes due to humidityvariation (Fig. 7d). We note that for a thicker f-CNT or goldcoating, no humidity response was observed.

f-CNT-SS annealing and actuator function. We observed thatan application of 100 mA current with a 40-s duration perma-nently reduced the resistance of f-CNT-SS by B15%(Supplementary Fig. S8). The result was conrmed on threeseparate f-CNT-SS bres. A systematic decrease in post-annealedresistance was observed up to 150 mA.By using the f-CNT coating as a local heating element, it was

possible to utilize a single f-CNT-SS bre as a current-driven

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Figure 4 | Raman spectra of f-CNT-SS bre and dry f-CNT powder.

The f-CNT-SS spectrum has been offset to show the linear background

contribution more clearly (blue: f-CNT-SS bre, red: dry f-CNT powder).

Inset shows background-subtracted and normalized spectrum of f-CNT-SS,

showing an overlap with the spectrum of the control sample.

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Figure 5 | Tensile properties of neat and processed SS single bres.

Measurement was performed at 25% RH to reduce the effects of water

vapour absorption on the tensile properties (dashed lines: neat bres,

circles: dry supercontracted bres, solid lines: f-CNT-SS bres).




actuator (Fig. 8). The actuator could be operated in two modes.First, the contraction mode (Fig. 8b) was achieved by applying a100-mA current to a single f-CNT-SS bre in its natural partially

hydrated state under ambient conditions (55% RH, 23 C). Thelocal heat (B5mW) generated by a 100-mA current dehydratedthe bre, therefore contracting the bre by B1% of its original

Wovenf-CNT-SScontact

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Figure 6 | Custom-shaped f-CNT-SS for clamped and woven electrodes. (a) Photographs of f-CNT-SS shaped into coil, ring, knotted and letter forms.

(b) Photograph of clamped (I) and woven (V) contacts used to characterize the temperature-dependent resistance of f-CNT-SS with clampI contact expansion. Scale bar: 500mm. (c) VRH nature (that is, ln(R)B(1/T)1/4) of f-CNT-SS.

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Figure 7 | Strain-dependent resistance of f-CNT-SS. (a) Strain-dependent resistance of a single f-CNT-SS bre up to 50% strain. (b) Cyclic strain test

of a single bre with a 1.2 gauge factor (red line: resistance, blue line: strain). (c) Demonstration of the strain sensitive resistance of f-CNT-SS to

heart pulses. A bundle of f-CNT-SS with RB11 kO was used in this device. Left and right scale bars correspond to 1 cm and 1mm, respectively. (d) Cyclic RHtest up to 70% RH of a 2 2mm2 f-CNT-SS mat with RB100O (red line: DR, blue line: RH).




length (1.8mm). Consequently, the mass (35mg) was lifted byB25 mm (Fig. 8b). The lifting action occurred almost instanta-neously (r1 s). After B5min, the bre relaxed close to itsoriginal length due to rehydration. A longer time was required tofully rehydrate. The slow relaxation rate may be due tothe vapour absorption rate intrinsic to the SS. Operation atlower temperature and/or higher RH may decrease the brerelaxation time.In contrast with the contraction mode, the extension mode was

achieved by applying a current after the initial dehydration. Herean application of current extended the bre by B0.15% of itsdried length, lowering the mass by B4 mm (SupplementaryMovie 3). Furthermore, both the bre extension and contractionactions occurred within 1 s. In principle, increasing the appliedcurrent would increase the extension length primarily due tothermal expansion. In both cases, we conrmed the repeatabilityof this behaviour for three different single f-CNT-SS bres.

DiscussionThe essential aspects leading to the realization of the f-CNT-SSmaterial are as follows. The f-CNTs are polar, with positive chargeat the amine sites. The SS is a protein-based polymer where theamino acid groups vary along the backbone, some are neutral andsome polar27. By mechanical mixing, the dry f-CNT powder ispartially dispersed and adheres to the SS bres due to polarinteraction (Supplementary Fig. S3). When water is applied to themixture, the f-CNTs disperse further and the SS bres experiencehydrogen bond breaking8, resulting in bre swelling and softening.As a result, the surface area of the bre is increased, allowing moref-CNTs to adhere to the bre. Applications of shear and pressurebring the f-CNTs in closer proximity to the surface of the bre,promoting both physical and chemical interactions between them.Upon drying, the SS bre matrix shrinks further as hydrogenbonding is re-established8,28, concentrating the CNT array andmaking it electrically conducting.The FTIR spectra in Fig. 3 indicate a change in the nature of

the SS carboxylic acids, consistent with the aqueous chemicalreaction between the NH2 side groups of the f-CNT and theCOOH component of aspartic and glutamic acids in the SS. Asour water-based method is performed at room temperature,amide formation is unlikely because this type of reactionnormally occurs at high temperatures22. However, ionic andhydrogen bonding are both likely to occur. In an aqueoussolution, for example with pH of 47, some amount of side-chaincarboxylic acids and amines are typically ionized. By applyingshear and pressure, ionic bonding between them is promoted. Inparallel, the NH2 side groups may form hydrogen bonding withthe non-ionized aspartic and glutamic acids (SupplementaryFig. S9). The formation of hydrogen bonding at roomtemperature has been observed in the fabrication of buckypaperfrom O-H-functionalised CNTs29. Even though the f-CNT

anchoring to the SS bre is minimal due to the smallabundance of aspartic and glutamic acids, it generates asignicant grafting, such that when combined with the van derWaals interaction and the natural tendency of the f-CNTs toentangle, a new hybrid functional silk bre is produced(Supplementary Fig. S9).The intimate adherence of the f-CNTs on the SS bre is the key

factor that results in many of the observed phenomena such asthe improved extensibility and toughness of the f-CNT-SS bre.The toughness of f-CNT-SS bres can be attributed rst to thesupercontraction that occurs during the coating process andsecond to the distribution of SS radial deformation by the f-CNTcoating. SS bres generally become tougher after being super-contracted. The release of internal pre-stress8 in neat SS bresduring supercontraction accounts for the additional energy theycan absorb before rupturing. The f-CNT coating may furtherimprove the toughness by effectively distributing the SS radialdeformations associated with the rupture point during theextension process. A SS bre experiences a very large radialdeformation when strained longitudinally, with a Poisson ratio (ameasure of how much the diameter of the bre shrinks when thebre is extended) of B1.5 (ref. 30). In contrast, a CNT network,such as the buckypaper, experiences signicantly less deformation(Poisson ratio up to 0.3), except in some special cases31. Ifintimately connected to the SS bre, this can reduce the radialdeformation of the silk bre at the highest rupture point, forexample, in the middle of the bre, allowing further extensionbefore it ruptures.The uniformity of the f-CNT coating is demonstrated by the

absence of sudden jumps in the resistance versus strain curve upto at least 50% strain (Fig. 5a). The uniform adherence strengthbetween the f-CNT to SS and f-CNT to f-CNT contacts allowshomogenous strain distribution during extension, similar to themechanism observed in CNT-elastomer or gold-elastomersystems32,33, where extensions of 4100 or 20% are observed,respectively. Additional exibility may be provided by the breshrinkage during the water-based processing, which effectivelyconcentrates the f-CNT network. We emphasize that no f-CNTcrosslinkers are used in our water-based coating method. Withoutcrosslinkers, it has been reported that the f-CNT network istypically very brittle23.The 3D VRH transport of the f-CNT-SS reveals that the

conductivity is dominated by inter-tube charge carrier hoppingbetween the f-CNTs34, which may be related to the submicronlength of the f-CNTs used (therefore resulting in a larger numberof inter-tube contacts). The T0 and conductivity of the f-CNT-SSand the benzoquinone-crosslinked buckypaper are similar.This suggests that the simple water-based coating method iseffective. The slightly higher T0 could mean that the f-CNTswere not as tightly entangled as in the case of crosslinkedbuckypaper, resulting in a slightly higher contact resistancebetween the f-CNTs.

Weight

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0 A 100 A 0 A

Drying contraction

100 A 0 A 100 A

Figure 8 | F-CNT-SS actuator activated by Joule heating. (a) Photograph of a 35mg mass hanging on a single f-CNT-SS bre. Scale bar: 250mm. (b) Cyclicf-CNT-SS contraction. The bre was allowed to relax for 1 h before the next current cycle to ensure full rehydration. Scale bar: 150mm.




The application of an annealing current under ambientconditions indicates a reduction in the contact resistance of thef-CNT network. A similar effect is observed in carbon nanobre(CNF)gold interconnects, where the application of an annealingcurrent at ambient condition reduces the contact resistancebetween the CNFgold interconnects35. As the f-CNT coating onthe SS surface is very thin, B80100 nm, a 100-mA annealingcurrent may generate a considerable current density that heats theCNTCNT joints, producing better contacts.Owing to the thin, exible and porous nature of the f-CNT

network, external stimuli such as varying strain and humiditylevels will affect the SS bre. We emphasize that for gold-coated(B20-nm thick) or thicker f-CNT coated SS (several micrometrethick; Methods section), and neat buckypaper (B30 mm thick),the humidity response is not observed.As a local heating element, the f-CNT coating allows us to

drive the SS-based actuator reported earlier11 using electricalcurrents in contraction and extension mode by exploiting theswelling/de-swelling and thermal expansion/contraction of thesilk bre, respectively. The f-CNT-SS contraction ofB1% in ourproof-of-concept demonstration (performed at 55% RH) iscomparable to the reported value in the previous study11. Intheir case, the average contraction for lifting a 9.5-mg mass isB1.7% in the full-humidity range of 9010% RH. Assuming alinear correlation between the contraction length and RH, weexpect that a variation of RH from 55 to 10% will generate anaverage contraction of 0.95%, which agrees very well with ourresult. This suggests that our coating approach does not degradethe actuating properties of the silk bre.In conclusion, we have developed a simple and effective water-

based and shear-assisted method of fabricating tough, versatile,exible and multi-functional f-CNT-SS bres. Amine-functiona-lised MWCNT adheres effectively to the SS bre, as revealed bySEM and TEM images and by structural changes in the carboxylicacid of the SS as observed in the FTIR spectra. The uniformity ofthe coating is further conrmed from the Raman spectra, strain-dependent resistance and electrical conductivity estimation. Thecharge carrier transport is primarily driven by inter-tube chargehopping, as revealed by the 3D VRH temperature-dependenttransport. The combination of a thin, exible and porous CNTnetwork with SS bres is synergistic, resulting in polar, custom-shapeable, self-monitoring and actuating devices.

MethodsCNTcoating. N. clavipes dragline silk bres were harvested from its natural habitatand used in the experiments. According to the product specications, theMWCNTs (NanocylTM NC3152) are submicron in length, functionalised withNH2 groups (o0.5 wt %),B10 nm in average diameter, and have carbon purity of495%. From the TEM image (Supplementary Fig. S10), it was observed that theamine-functionalised MWCNT (hereafter f-CNT) is composed of B17 nanotubelayers. The f-CNT coating method is outlined in Fig. 1. Dry f-CNT powderand SS bres were rst mixed mechanically. The pre-coated bres were wet by fewdrops of water on a Teon substrate. The wet bres were pressed and shearedbetween two Teon substrates, followed by air drying under ambient conditions.We also obtained f-CNT-SS with a thicker coating (510 mm, SupplementaryFig. S11) by dipping the neat bres into a benzoquinone/f-CNT solution20 anddrying at 50 C, repeatedly 2030 times. However, in addition to the more complexpreparation, the bres produced with this method (compared with the water-basedprocedure) were not as extendable. The benzoquinone/f-CNT solution was alsoused to make the buckypaper control sample.

TEM and SEM imaging. Bundles of f-CNT-SS bres were rst xed with 3%glutaraldehyde in water solution (diluted from 50% glutaraldehyde in water solu-tion purchased from Electron Microscopy Sciences) for 2 hours at room tem-perature. The bres were then dehydrated in ethanol, followed by embedding inEpo-x (5:25 hardener/resin weight ratio, Electron Microscopy Sciences) at 50 Cfor 4 h. A Leica Ultracut E microtome was used to obtain 90-nm thick sections off-CNT-SS embedded in epoxy that were then transferred onto TEM 100 meshcopper grids. TEM images were obtained with a cold eld-emission JEOL JEM-

ARM 200F at 80 keV. SEM images were obtained by eld-emission JEOL JSM-7410F SEM at 5 keV.

FTIR spectroscopy. FTIR spectroscopy was performed under attenuated totalreectance (ATR)-FTIR mode using the Smart Orbit diamond ATR accessoryof the Thermo Scientic Nicolet 6700 spectrometer. The measurements werecarried out in absorption mode between 4,000 and 400 cm 1 with 2 cm 1resolution at 23 C and B25% RH. The same silk bres were used for each set ofmeasurements, for example, before and after treatment or CNT coating. Beforeeach measurement, the samples were stored under low-humidity conditions using aPetri dish with a desiccant.

Raman spectroscopy. Raman spectroscopy was performed using a RenishawInvia micro-Raman with a 785-nm excitation wavelength, 3mW power, 50magnication, 20 s exposure and two accumulations. The samples were mountedon a gold substrate to reduce undesired background Raman signals.

Tensile properties. Stressstrain measurements of single silk bres were carriedout using Dynamical Mechanical Analyser (DMA, Q800, TA Instruments) instrain-ramping mode. The measurements were performed at a 2% per min strainrate under 23 C and 25% RH. Prior to each stressstrain measurements, thediameter of each single silk bre was determined using an optical microscope( 500 magnication) with dark eld illumination. A lithographically patternedscale with 1-mm resolution was used as a reference. The single silk bre wasmounted between two Kapton pads connected by a thin Kapton bridge. The brewas secured using a hard 2850 Stycast epoxy (cured at 23 C for 24 h). After thepads were clamped into the DMA, the bridge was cut. To ensure reliability, the dataobtained from bres that broke near the clamp edges were not used.

Resistivity measurement. The electrical conductivity of the single f-CNT-SS brewas estimated on a 1.2-cm long sample in four-terminal conguration with carbonpaste electrical contacts. The distance between the inner electrodes was 2mm(RB810 kO). Typical geometrical factors were used in the estimation, for example,the supercontracted SS bre diameter of B6.5 mm (Fig. 2a and SupplementaryFig. S1) and f-CNT coating of B80100 nm (Fig. 2e and Supplementary Fig. S2).Using these values, the cross-section area of the annular CNT coating wasestimated to be between 1.6 and 2.04 mm2. The electrical conductivity of thebuckypaper control sample was estimated using a sample with length, width,thickness and resistance of 0.276mm, 172.5 mm, 30 mm and 41O, respectively. Theresistance was measured using a Keithley 6221 current source and a Keithley2182A nanovoltmeter in a four-terminal conguration.

Strain-dependent resistance. A Parker Actuator 401XR translational stage with aCompax3S Servo Drive System was used to exert strain on the sample at0.3 mms 1 rate. Single bres or bundles of f-CNT-SS were mounted in a four-terminal conguration using carbon paste.

f-CNT-SS for heart pulse monitoring. f-CNT-SS bundle was mounted in a four-terminal resistance conguration using carbon paste on a substrate with a gapbetween the voltage leads to allow the f-CNT-SS bundle to bend during mon-itoring. The carbon paste contacts were protected by enamel coating.

Humidity-dependent resistance. Single bres or bundles of f-CNT-SS weremounted on a G10 substrate in four-terminal conguration using carbon paste andwere placed inside a custom-made humidity chamber.

f-CNT-SS actuator. A single f-CNT-SS bre was mounted between two G10substrates in a two-terminal electrical conguration using carbon paste. A piece of35mg solder wire with a 24-mm gold wire hook was used as a hanging weight.Current was applied with a Keithley 6221 current source and the f-CNT-SSmovement was recorded with an optical microscope equipped with a digitalcamera. Video analysis was performed to obtain the displacement behaviour of thef-CNT-SS.

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AcknowledgementsThis work was supported in part by NSF-DMR 1005293 and 1105129, and carried out atthe National High Magnetic Field Laboratory, supported by the NSF, the DOE and theState of Florida. W.R.S. thanks Fulbright Visiting Scholar 2011 program. The Barcelonagroup thanks MICINN (CTQ2010-19501) and CIBER-BBN, an initiative funded by theVI National R&D Plan 20082011, Iniciativa Ingenio 2010, Consolider Program. Wethank Drs Yi-Feng Su and Xixi Jia for assistance in the TEM study. We would also like tothank Dr Jin Gyu Park for the assistance in the tensile measurement and Raman spec-troscopy study. We are grateful for the access to DMA and Raman spectroscopy facilitiesprovided by HPMI.

Author contributionsE.S., W.R.S. and S.F.A.A. conceived the original idea and performed preliminaryexperiments. V.Le. and V.La. characterized the strain-dependent resistance. R.G.A.and E.S. performed the FTIR spectroscopy and analysis. E.S. performed the restof the experiments. E.S. and J.S.B. wrote the manuscript and contributed to projectplanning.

Additional informationSupplementary Information accompanies this paper at http://www.nature.com/naturecommunications

Competing nancial interests: The authors declare no competing nancial interests.

Reprints and permission information is available online at http://www.nature.com/reprintsandpermissions/

How to cite this article: Steven, E. et al. Carbon nanotubes on a spider silk scaffold.Nat. Commun. 4:2435 doi: 10.1038/ncomms3435 (2013).

This work is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 3.0 Unported License. To view a copy of

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title_linkResultsWater-based f-CNT coating of SS fibresFourier transform infrared spectroscopy

Figure1f-CNT-SS processing steps.A neat SS bundle is mixed with a dry f-CNT powder. The mixture is then wet, pressed, sheared between two PTFE substrates, and air dried. Scale bar: 2thinspmmRaman spectroscopy

Figure2Single f-CNT-SS fibre surface profiles.(a) SEM image of f-CNT-SS. The diameter of the fibre is sim6.5thinspmgrm. Scale bar: 10thinspmgrm. (b,c) Magnified SEM images of f-CNT-SS surface showing a uniform mat-like covering. Scale bars: 1thinspmgrFigure3FTIR spectra of neat and f-CNT-SS fibres.(a) Combined FTIR spectra normalised to the amide I peak at 1,625thinspcm-1 (red: neat fibres, blue: f-CNT-SS). (b) Expanded view of the spectra between 3,000 and 2,800thinspcm-1. Arrows indicate the posiTensile propertiesElectrical conductivityCustom shaped f-CNT-SSTemperature-dependent resistanceStrain-dependent resistancef-CNT-SS annealing and actuator function

Figure4Raman spectra of f-CNT-SS fibre and dry f-CNT powder.The f-CNT-SS spectrum has been offset to show the linear background contribution more clearly (blue: f-CNT-SS fibre, red: dry f-CNT powder). Inset shows background-subtracted and normalizedFigure5Tensile properties of neat and processed SS single fibres.Measurement was performed at 25percnt RH to reduce the effects of water vapour absorption on the tensile properties (dashed lines: neat fibres, circles: dry supercontracted fibres, solid liFigure6Custom-shaped f-CNT-SS for clamped and woven electrodes.(a) Photographs of f-CNT-SS shaped into coil, ring, knotted and letter forms. (b) Photograph of clamped (IPlusMinus) and woven (VPlusMinus) contacts used to characterize the temperature-depFigure7Strain-dependent resistance of f-CNT-SS.(a) Strain-dependent resistance of a single f-CNT-SS fibre up to 50percnt strain. (b) Cyclic strain test of a single fibre with a 1.2 gauge factor (red line: resistance, blue line: strain). (c) DemonstratiDiscussionFigure8F-CNT-SS actuator activated by Joule heating.(a) Photograph of a 35thinspmg mass hanging on a single f-CNT-SS fibre. Scale bar: 250thinspmgrm. (b) Cyclic f-CNT-SS contraction. The fibre was allowed to relax for 1thinsph before the next current cMethodsCNT coatingTEM and SEM imagingFTIR spectroscopyRaman spectroscopyTensile propertiesResistivity measurementStrain-dependent resistancef-CNT-SS for heart pulse monitoringHumidity-dependent resistancef-CNT-SS actuator

RobinsonB. H.E-waste: an assessment of global production and environmental impactsSci. Total Environ.4081831912009YangY.Toughness of spider silk at high and low temperaturesAdv. Mater.1784882005AllmelingC.JokusziesA.ReimersK.KallS.VogtP. M.Use of spider sThis work was supported in part by NSF-DMR 1005293 and 1105129, and carried out at the National High Magnetic Field Laboratory, supported by the NSF, the DOE and the State of Florida. W.R.S. thanks Fulbright Visiting Scholar 2011 program. The Barcelona grACKNOWLEDGEMENTSAuthor contributionsAdditional information

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